1. Technical Field
The invention relates to the art of making partially stabilized zirconia with yttria and, more particularly, to the technology of eliminating the inherent aging degradation of such ceramic.
2. Description of the Prior Art
It is well known in the art that transformation toughened, partially stabilized zirconia (PSZ) ceramics, in which the stabilizer is yttria, have a major defect which comprises deterioration with elapse of time in the temperature range of 200.degree.-300.degree. C.; the tetragonal phase zirconia in the surface layer, typical of the toughened material, reverts to the stable monoclinic form and the strength is substantially diminished. The deterioration is exacerbated by water. A number of explanations of this phenomenon have been offered but general agreement has not been achieved.
Although dealing with partially stabilized zirconia solid electrolytes, which are not transformation toughened, U.S. Pat. Nos. 4,360,598 and 4,370,393 do address the deterioration problem stated above. Since the materials are not toughened, the stable phases present at room temperature are monoclinic and cubic. These non-toughened materials are produced by firing at temperatures of at or above 1500.degree. C., at which temperature the stable phases are tetragonal and cubic or purely tetragonal, depending on the yttria content. With prolonged heating, grain growth will occur; on cooling, the tetragonal phase converts to the monoclinic or monoclinic and cubic phases, and the tetragonal to monoclinic transformation is accompanied by a volume expansion of about 4%. According to the aforementioned patents, this expansion on cooling due to crystal transformation leads to mechanical stress at the grain boundaries which, in turn, leads to cracking and deterioration when the material is held at temperatures of 200.degree.-300.degree. C. for extended periods. These patents further state that the problem can be overcome or reduced if the grain size is less than 2.mu., so that fine grained materials are protected from deterioration.
Transformation-toughened PSZ, containing Y.sub.2 O.sub.3 as a stabilizer, will contain only tetragonal phase zirconia at room temperature if the Y.sub.2 O.sub.3 content is less than about 4 mol percent, and a mixture of tetragonal and cubic phases with more than about 4 mol percent Y.sub.2 O.sub.3. The high temperature tetragonal phase is retained at room temperature by firing a very reactive high surface area coprecipitated ZrO.sub.2 --Y.sub.2 O.sub.3 powder, in the temperature range of 1400.degree.-1500.degree. C., so as to obtain a fine-grained ceramic body. Since this material contains no apparent monoclinic phase, it appears at first glance as if the deterioration mechanism, of crystal transformation stress, suggested by U.S. Pat. Nos. 4,360,598 and 4,370,393, cannot be applicable. However, in order to form a useful item from a fired PSZ body, machining of some sort is usually required and this operation causes the transformation of some of the tetragonal zirconia in the machined surfaces to the monoclinic form, so that stress associated with the formation of the monoclinic phase again presents the problem of low temperature aging deterioration.
An article entitled "Degradation During Aging of Transformation-Toughened ZrO.sub.2 --Y.sub.2 O.sub.3 Materials at 250.degree. C.', by Lange et al, appearing in the Journal of the American Ceramic Society, Vol. 69, pp. 237-240 (1986), deals directly with the use of low temperature aging deterioration in transformation-toughened ZrO.sub.2 --Y.sub.2 O.sub.3 materials and suggests that water leaches yttrium out of tetragonal PSZ grains, leading to their transformation to the monoclinic form accompanied by expansion which leads to stress and, if the monoclinic grains are above a critical size, microcracking. In any event, the connection between the deterioration and the presence of the monoclinic crystalline phase is generally invoked. That the deterioration mechanism suggested by Lange et al further requires the involvement of water is not a drawback, since water vapor is always present in ambient atmospheres.
Even if process steps are taken to ensure the inhibition of the transformation of tetragonal crystal to the monoclinic crystal, such as suggested in U.S. Pat. Nos. 4,370,393 and 4,360,598, there remains the stimulation of the monoclinic phase by machining of such sintered material. That is to say, the material may be cooled to room temperature by observing certain sintering conditions so that the tetragonal crystal phase predominates even at room temperature, accompanied only by small seeds of monoclinic crystal.
The important problem that remains is that of eliminating the monoclinic phase from a partially stabilized zirconia material after sintering whether stimulated by machining or by improper sintering techniques.